I think you are being both irrational and insensitive towards your husband, and looking to place the 'blame' somewhere so that you can feel a little 'in control' of your current situation
. Have you heard the phrase ''flogging a dead horse''?......
No-one actually believes smoking/ drinking/ drugs/ fast driving/ bungee jumping/ crossing busy roads without using the crossing 200 yds away etc etc, will really KILL them do they ?? If they did, then no-one would do it, would they ? Like, ever ! Everyone truly believes that all the bad stuff, cancer/ dying young etc only happens to OTHER people ! 
It's a huge shock to realise that bad things can, and do, happen to YOU and YOUR loved ones
Unfortunately thousands of people will die every year, through no apparent fault of their own.
And some will have people in their life who will say 'it's your own fault ! If only you didn't smoke/ drink/ bungee jump/ rock climb etc'' But that will not help anyone, will it ??
OK, so your DH hasn't managed to stop smoking entirely, but he DID manage to cut down very drastically to please you, and has probably gone weeks/ months without smoking at all for years. All of which has no doubt kept him healthier than he might otherwise have been.
You're blaming your DH because, in your grief, you NEED to try to find SOMEONE to blame, but it's not his fault
You'll only alienate him if you blame him, and at the time when he needs you the most, and I'm sure you would hate to do that to him.
Tell him he has your full love and support, and if he says anything at all about smoking, just tell him that it doesn't matter how he became ill, it just matters how you are all going to get through it as a family.
(and remember, 1 in 2 people will get cancer at some point in their lives, and MOST of them won't ever have smoked )
I'm wishing you both the very best 